High power, long focus electron source for beam processing

ABSTRACT

Beam processing methods including e-beam welding and e-beam evaporation for thin film deposition are implemented with a novel high power, long focus electron source. The high power, long focus electron source generates an e-beam. The e-beam is transported through a series of steering magnets to steer the beam. At least one refocusing magnet is provided to refocus the e-beam. A final steering magnet bends the e-beam to focus on a target, such as a weld joint or a deposition target.

This application claims the benefit of U.S. Provisional Application No.60/611,629, filed on Sep. 21, 2004.

CONTRACTUAL ORIGIN OF THE INVENTION

The United States Government has rights in this invention pursuant toContract No. W-31-109-ENG-38 between the United States Government andArgonne National Laboratory.

FIELD OF THE INVENTION

The present invention relates to novel applications of the inventiondisclosed in U.S. Ser. No. 10/887,142 filed Jul. 8, 2004(ANL-IN-04-039), entitled FIELD EMISSION CATHODE GATING FOR RF ELECTRONGUNS AND PLANAR FOCUSING CATHODES which embodied a method forimplementing a unique electron gun, thus allowing high quality electronbeams with high repetition rates to be produced. More particularly, thepresent invention relates to beam processing methods, including e-beamwelding and e-beam evaporation for thin film deposition implemented witha high power, long focus electron source.

DESCRIPTION OF THE RELATED ART

Currently, electron beam welding is performed with electron sources of15-50 kV DC and several amps emission. The electron guns have shortfocal lengths, and are often stationary or have limited motion. Theseconstraints reduce the efficacy of e-beam welding, for example, the weldmust be initiated on the outside of the part requiring full beampenetration of the weld joint. Such a process can leave trapped voids onthe inner surface of the part, which can trap contamination and producesa sub-optimal weld.

A common method for thin film deposition is laser ablation deposition,which is used to deposit alloys and compounds on a variety of surfaces.Laser ablation deposition is being used increasingly to deposit alloysand compounds. Laser ablation has better control over alloy compositionin the deposited film as compared to sputter or thermal vapor depositiontechniques. The laser ablation has the advantage that there is a singleenergy source to control. Although laser ablation uses the single, andtherefore easily controllable, energy source, the ablation processcannot be well controlled. The shot to shot ablation can be different.

E-beam evaporation creates a melt pool from which material evaporates.The amount of material that evaporates is more uniform creating a moreuniform film as it results from evaporation from a liquid instead ofablation from the surface. However, due to the short focal length ofexisting e-beam evaporators, multiple sources are needed for alloy andcompound deposition.

Most radio frequency (RF) electron guns constructed to date use eitherthermionic cathodes or photocathodes as their electron sources.Thermionic cathodes, which use high temperatures to induce electronemission from the cathode material, constantly emit electrons wheneverthe electric field in the gun is in the correct phase to accelerateelectrons away from the cathode. Photocathodes use a light source,typically a high-power laser, to extract electrons from the photocathodesurface.

Thermionic-cathode RF electron guns can typically produce very highaverage power electron beams, because of the continuous nature of theelectron emission from the cathode, but can suffer from degraded beamquality because the electron emission cannot be gated to a particularfraction of an RF period. In addition, due to the requirements for hightemperatures (ca 1300 C), thermionic cathodes are generally unsuited foruse in superconducting RF electron guns (which generally requireoperating temperatures around four degrees above absolute zero).

Photocathode RF electron guns can produce very high-quality (bright)electron beams, because the use of a laser allows electron emission tobe gated to a specific portion of the RF period, but most drive laserscannot produce a laser pulse at every RF period. Therefore, the averagebeam power is typically lower than for a comparable thermionic-cathodeRF electron gun. Photocathodes in common use typically offer a choicebetween either long lifetime and poor efficiency thus requiring a farlarger drive laser, or poor lifetime and high efficiency requiring theuse of a large cathode fabrication and processing system adjacent to theelectron gun.

Field emission cathodes have generally not found widespread use in RFelectron guns because they will, all other things being equal; emit themost charge when the applied electric field is highest. This isgenerally not the most desirable time for electron emission, and wouldresult in a very poor-quality beam.

A concave cathode surface can be used for focusing an electron beam forRF electron guns. This approach, however, has two primary disadvantages.First, the focusing thus provided is fixed; for any reasonable cathodedesign, altering the radius of curvature in situ while maintaining thesurface quality required to support high RF field strengths does notappear to be practical. Second, because the cathode is curved, unless aspecially prepared drive laser is used, electron emission will start atthe edges of the cathode before the center, and will likewise end at theedges of the cathode before ending at the center. These two effects arethe primary reason such techniques are not more widely used in existingelectron gun designs. In particular, the inability to alter the radiusof curvature of the cathode, in effect the focusing force has been seenas a strong disadvantage.

Principal objects of the present invention are to providebeam-processing methods implemented with a novel high power, long focuselectron source, including e-beam welding and e-beam evaporation forthin film deposition.

Other important objects of the present invention are to provide suchbeam processing methods substantially without negative effect and thatovercome some disadvantages of prior art arrangements.

SUMMARY OF THE INVENTION

In brief, beam processing methods including e-beam welding and e-beamevaporation for thin film deposition are implemented with a novel highpower, long focus electron source. The high power, long focus electronsource generates an e-beam. The e-beam is transported through a seriesof steering magnets to steer the beam. At least one refocusing magnet isprovided to refocus the e-beam. A final steering magnet bends the e-beamto focus on a target, such as a weld joint or a deposition target.

In accordance with features of the invention, the electron source has alarge focal length. The long focal length of the invented electronsource enables a single electron source to be used. In addition, withthe e-beam advantageously steered via magnets enables the beam to betransported inside a target part, allowing welds to be made from theinside of the part, eliminating trapped voids.

In accordance with features of the invention, the invention is used tovapor deposit coatings on the interior of the target part that isimplemented with an identical electron gun and using the same steeringand focusing magnets as used for welding. The e-beam is focused on atarget inside a component to evaporate material and coat the inside ofthe part. Thus, both the benefits of e-beam evaporation and a singleenergy source are realized. This e-beam evaporation process is at leastas effective and more controllable than known conventional processes.

In accordance with features of the invention, the steering magnetsinclude dipole magnets and the refocusing magnets include quadrupolemagnets. The final bending or steering magnet includes, for example, adipole magnet.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention together with the above and other objects andadvantages may best be understood from the following detaileddescription of the preferred embodiments of the invention illustrated inthe drawings, wherein:

FIG. 1 is a schematic diagram illustrating an exemplary 1-cell RFelectron gun for implementing methods in accordance with the presentinvention;

FIG. 2 is a chart illustrating beam emission timing for the RF electrongun of FIG. 1;

FIG. 3 illustrates the FE cathode emission times during the RF period ofFIG. 2 for implementing methods in accordance with the presentinvention;

FIGS. 4A and 4B illustrate the two lowest modes or field patterns forthe single-cell RF electron gun of FIG. 1 in accordance with the presentinvention;

FIGS. 5A, 5B, and 5C illustrate the effect of adding a third-harmoniccomponent to the fundamental, at a particular point in the RF cavity, asa function of the phase of the fundamental field in accordance with thepresent invention;

FIGS. 5D, 5E, and 5F illustrate the effect of subtracting athird-harmonic component to the fundamental, at a particular point inthe RF cavity, as a function of the phase of the fundamental field inaccordance with the present invention;

FIGS. 6A, 6B, and 6C illustrate the effect of adding a third-harmoniccomponent to the fundamental, at a particular point in the RF cavity, asa function of the phase of the fundamental field for another selectedproportionality constant and phase of the 3rd harmonic field inaccordance with the present invention;

FIG. 7 illustrates an exemplary FE cathode emission profile during theRF period of FIGS. 6A, 6B, and 6C for implementing methods in accordancewith the present invention;

FIG. 8 illustrates FE cathode gun cell test geometry used forsimulations in accordance with the present invention;

FIG. 9 illustrates the fundamental and 3rd harmonic fields strengthplotted as distance along the axis of the gun of FIGS. 1 and 8 inaccordance with the present invention;

FIG. 10 illustrates an exemplary application of the gun of FIGS. 1 and 8in accordance with the present invention;

FIG. 11 is a detailed view of the RF electron gun of FIG. 1 illustratinga novel planar focusing cathode that provides a focused electron beam inaccordance with the present invention;

FIG. 12 illustrates exemplary electric field contours of the planarfocusing cathode of FIG. 11 in accordance with the present invention;

FIG. 13 illustrates exemplary normalized radial electric field at thecathode surface of the planar focusing cathode of FIG. 11 in accordancewith the present invention; and

FIG. 14 is a schematic diagram illustrating an exemplary beam processingapparatus including the RF electron gun of FIG. 1 for implementing beamprocessing methods in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with features of the invention, beam processing methodsincluding e-beam welding and e-beam evaporation for thin film depositionare implemented using a novel high power, long focus electron source.The electron source includes a radio frequency (RF) electron gun with afield-emitter cathode providing a focused electron beam and implements ageneral method for altering the emission time of a field-emitter cathodewith respect to the RF period in the gun. This approach combines theadvantages of the thermionic-cathode RF electron gun (beam producedevery RF period, no laser needed) with those of a photoinjector (gatedemission at the most desirable time, high brightness, superconductingRF-compatible). The resulting high power, long focus electron sourceenables broad applicability across a number of fields.

In accordance with features of the novel high power, long focus electronsource, a planar focusing cathode, also referred to as a standoffcathode, provides a means of focusing an electron beam emitted from thecathode of a high-brightness RF electron gun, without requiring the useof either magnetic fields, or a curved cathode surface.

In accordance with features of the invention, beam-processing apparatusfor implementing beam processing methods is illustrated and describedwith respect to FIG. 14. In FIGS. 1-13, the novel high power, long focuselectron source used for implementing beam processing methods isillustrated and described and is disclosed in U.S. patent applicationSer. No. 10/887,142 by the present inventors filed Jul. 8, 2004(ANL-IN-04-039), entitled FIELD EMISSION CATHODE GATING FOR RF ELECTRONGUNS AND PLANAR FOCUSING CATHODES, and assigned to the present assignee.The subject matter of the above-identified patent application isincorporated herein by reference.

Having reference now to the drawings, in FIG. 1 there is shown anexemplary RF electron gun generally designated by the referencecharacter 100 that can be used for implementing methods in accordancewith the present invention. The RF electron gun 100 is a single cell or1-cell RF electron gun that is essentially a box in which an oscillatingelectromagnetic field is generated. The RF electron gun 100 includes acathode 102 that provides an electron source. The RF electron gun 100includes an RF power feed 104 that is used to establish an oscillatingelectromagnetic field inside a resonant cavity 106. This field is usedto accelerate electrons emitted from the cathode 102 out a beam exitiris 108 provided within conducting walls 110 defining the resonantcavity 106.

In general, RF electron guns work by establishing an oscillatingelectromagnetic field inside a cavity, or series of cavities, such asresonant cavity 106 defined by conducting walls 110. This field is usedto accelerate electrons emitted from a cathode, down the bore of thegun, and out an exit port, such as from cathode 102 and out beam exitiris 108. The phase of the RF field at which a given electron is emittedfrom the cathode 102 determines whether it can exit the cavity 106, and,if so, at what energy. Electrons attempting to leave the cathode 102 tooearly in phase, before the so-called zero-crossing, cannot exit thecathode at all because the electric field in the cavity is the wrongsign. Electrons emitted too late in phase cannot exit the RF electrongun 100 before the electric field reverses sign; these electrons will bedecelerated before they can exit the gun. This can cause the overallelectron beam quality to suffer. Electrons emitted still later will havetheir direction of flight reversed, and will return to strike somewherein the vicinity of the cathode. This phenomenon is calledback-bombardment.

FIG. 2 illustrates, for a single RF period, the fate of emittedelectrons as a function of phase including a plurality of regionsrespectively labeled 1, 2, 3 and 4. Although the exact dependence ofelectron beam quality and energy as a function of launch phase dependsvery strongly on details of the gun design and construction, somegeneral features can be identified. Electrons emitted during region 1,between 0 degrees and (approximately) 60 degrees, will exit the gun withreasonable beam quality and comparatively high electron beam energy.Electrons emitted during region 2 will exit the gun, but with greatlydegraded beam quality and lower beam energy.

Generally, there is a smooth transition between regions 2 and 3, fromwhich electrons will be emitted from the cathode but which will not beable to exit the gun; many of these electrons, in fact, will reversedirection and strike the cathode. Finally, in region 4, electrons cannotbe emitted from the cathode because the electric field on the cathode isthe wrong sign. Other properties of the beam, such as divergence, willchange smoothly with emission phase.

Having reference to FIG. 2, this provides a ready illustration of therelative advantages and disadvantages of photocathodes vs. thermioniccathodes. Thermionic cathodes emit electrons continuously, and so cangenerate very high average power electron beams; however, much of theiremission occurs during regions 2 and 3. Electrons emitted during region3 are by definition not relevant to the final electron beam quality, butthey still take energy to accelerate. The entire beam consists of amixture of electrons emitted during region 1 and region 2, resulting ina considerably lowered overall beam quality.

Photocathodes emit electrons only when struck with an appropriate pulseof light, as from a drive laser. Thus, it is possible to gate theelectron emission to only a very narrow slice within region 1, yieldinga very high-quality electron beam. The drive laser, however, addsconsiderable cost and complexity to the system, and cathode materiallimitations appear, at the present time, to prohibit bothhigh-duty-cycle and highly robust operation.

Finally, field emission (FE) cathodes operate by using strong electricfields to pull electrons from the cathode material directly. Thus,unlike thermionic cathodes, they do not emit continuously. Unlikephotocathodes, their triggering mechanism does not rely on an externalevent such as the arrival of a laser pulse. Rather, FE cathodes do notemit electrons below a threshold electric field. Above that threshold,which can be varied significantly depending on the cathode design, FEcathodes will begin to emit electrons, with the emission currentincreasing rapidly with increasing electric field.

At first glance, this behavior would seem to make FE cathodes a veryappealing alternative to both thermionic and photocathodes. Thedifficulty, however, lies in that the FE cathode will emit the highestcurrent when the electric field gradient is the strongest; the emissionwill be symmetric about the 90° point.

FIG. 3 illustrates the FE cathode emission times during the RF period ofFIG. 2. It is apparent that the FE cathode will emit most of its beamcurrent during region 2 of the RF period. The resulting beam willtypically have a very large energy spread, and very poor transversequality. Worse, some of the beam, emitted during region 3, may return tothe cathode and damage it via the back-bombardment process. In general,the emission from the FE cathode will be too long in duration, and willoccur at the wrong part of the RF period.

In fact, this description applies very well to the dark current observedduring the operation of some high-field RF photocathode guns, so namedas it describes electrons emitted without the presence of a drive laserpulse. In these cases, imperfections on the photocathode surface act asFE cathodes.

The resulting beams are typically low energy, with large energy spreadsand exceedingly poor transverse beam quality.

Potential mechanisms for addressing some of these shortcomings, such asshortening the cell containing the cathode, do not provide sufficientimprovement so as to make the FE cathode a viable choice for RF electronguns.

A given RF cavity is typically capable of supporting many differentfield patterns oscillating at many different frequencies. A specificpattern at a specific frequency is usually identified as a cavity mode.

FIGS. 4A and 4B illustrate the two lowest modes, or field patterns, forthe same single-cell RF electron gun. Typically, RF electron guns aredesigned to operate using a single mode in the cavity. Typically, the RFelectron gun is designed to use the lowest-frequency, or fundamental,mode. In this case, the two modes shown are the two lowest frequenciesthe cavity is capable of supporting. The higher frequency of FIG. 4B isnot an exact harmonic or integer multiple of the lower frequency of FIG.4A. Plot axes of FIGS. 4A and 4B are r (radius) vs. z (axial)coordinates. The arrows represent the direction and strength of theelectric field in the cavity.

It is possible to tune a cavity such that at least some of the modes areharmonic. For instance, it is possible to tune the cavity such that thethird cavity mode oscillates at exactly three times the frequency of thefundamental mode. The fields in the cavity will then beat in phase witheach other. (Some work has been performed using such a field sum togenerate what appears to be a flat cavity field. The thrust of the priorwork, however, had been to generate approximately uniform fields inspace rather than in time.)

FIGS. 5A, 5B, and 5C illustrate the effect of adding a third-harmoniccomponent to the fundamental, at a particular point in the RF cavity, asa function of the phase of the fundamental field.

FIGS. 5D, 5E, and 5F illustrate the effect of subtracting athird-harmonic component to the fundamental, at a particular point inthe RF cavity, as a function of the phase of the fundamental field.

At first glance, this does not appear to be particularly useful in that,although we can evidently control the duration of the peak field (expandinto a flat-top as in FIG. 5C, or sharpen as in FIG. 5F), the peak fieldis still centered squarely in region 2.

The field addition can be represented as follows:E _(sum)(t)=E ₁ sin(ω₁ t+φ₁)+E ₃ sin(3ω₁ t+φ ₃)  (1)where ω₁ represents the angular frequency of the fundamental field, E₁,E₃ represents the respective amplitude of the fundamental field and the3^(rd)-harmonic field, φ₁, φ₃ represents the respective phase of thefundamental field and the 3^(rd)-harmonic field, and t is time. We canchoose to set φ₁=0, and we can also write E₃=αE₁ where α is simply aproportionality constant. In FIGS. 5A, 5B, and 5C, in effect, α= 1/9,and φ₃=0° for case of addition, and in FIGS. 5D, 5E, and 5F α= 1/9, andφ₃=180° for case of subtraction.

Referring now to FIGS. 6A, 6B, and 6C, α and φ₃ may be set to bewhatever values desired. FIGS. 6A, 6B, and 6C illustrate the effect ofadding a third-harmonic component to the fundamental, at a particularpoint in the RF cavity, as a function of the phase of the fundamentalfield for another selected proportionality constant set to α=0.4 andphase of the 3rd harmonic field of φ₃=−40°.

Two features can be seen having reference to FIGS. 6A, 6B, and 6C.

First, the width of the peak field has narrowed considerably, comparedto the fundamental alone. Second, and most importantly, the peak of thefield has shifted from 90° to approximately 50°. Therefore, with anappropriately chosen emission threshold, the FE cathode will emitelectrons around the 50° point, within region 1.

FIG. 7 illustrates an example calculated emission profile for the fieldsshown in FIGS. 6A, 6B, and 6C.

In accordance with features of the invention, by adjusting the phase andstrength of the 3rd harmonic field relative to the fundamental field, wecause a field emission cathode to emit electrons at times appropriatefor the generation of high-brightness electron beams. The emission timeis gated by the combined fields and the response of the FE cathode tothe combined fields, much as a photocathode's emission is gated by itsdrive laser. Like a thermionic cathode, the FE cathode's emission is notdetermined by the presence or absence of a laser pulse; therefore, thecathode will produce beam at every RF period.

Therefore, this technique of the invention permits the combination ofappropriately gated emission, for high-brightness beam production, withemission during every RF period, for high-average-power operation. Thissummation of fields in the cavity represents, in effect, the first twoterms of a Fourier series describing an ideal driving field for afield-emission cathode gun. In principle, additional improvements to thefield shape could be made, for example, generating a small flat-topdistribution, by adding more fields at higher harmonics. In practice,this rapidly becomes less practical for two important reasons.

First, a reasonable method is required for coupling the harmonic powerinto the cavity, along with a suitable high-power microwave source. Forthe style of cavity, such as cavity 106 illustrated in FIG. 1, couplingpower into the cavity via an on-axis coupler at the exit of the gun 100can be provided together with using a cathode stalk or recess region asanother input coupler. Adding another harmonic requires another couplingport, for example, machined to even higher precision due to the higherfrequency, isolated from the first two harmonics. The same problems mustbe solved again for each successive harmonic added.

Second, the cavity 106 must be resonant at all harmonic frequencies inorder to build up reasonable field strengths. For the lowest cavity modeor the fundamental mode, the cavity radius is the dominant factor indetermining resonant frequency. For all other modes, both the cavityradius and the length are important in determining the resonantfrequency.

Therefore, to an extent, with two harmonics one can set the radius ofthe cavity to tune for the desired fundamental, and then adjust thelength to tune in the 3rd harmonic. This solves the resonant frequencyproblem without resulting to highly speculative cavity designs.

It should be understood that while it may be possible in principle toadd still higher harmonic fields to the cavity, and while this may be ofsome benefit, the primary concept of gating the field-emission cathodeto a useful beam launch time does not depend on doing so. Also by addingthe 5th harmonic component, rather than the 3rd harmonic component, doesnot offer any obvious advantages in terms of beam quality, and resultsin more peaks in the field sum. The result is that emission is not ascleanly gated to the desired time; instead, emission can occur atmultiple times during the fundamental RF period, leading to the risk ofcontaminating the desired beam.

There are additional considerations to be addressed in order to applythis technique of the invention to produce a viable electron beamsource. In particular, in order to obtain the properly gated electronemission as noted above, the 3rd harmonic field has to be quite strongin comparison to the fundamental field. For good beam dynamics in thegun, however, the fundamental field must dominate as the beam moves fromthe cathode to the exit. The addition of a modest 3rd harmonic field canbenefit beam transport, however, the required phase and amplitudes ofthe 3rd harmonic are shown with respect to FIGS. 5A, 5B, 5C, which isunsuitable for FE cathode gating.

A method is therefore required to obtain a strong 3rd harmonic fieldcomponent at the cathode 102, while minimizing its effects elsewhere inthe cavity 106. This is accomplished as follows. The gun cavity 106contains a recess where the cathode would ordinarily be, for example, asillustrated in FIGS. 8 and 11. The FE cathode is placed on a stalkrecessed slightly into this cavity. The 3rd harmonic field willpenetrate into the recess more deeply than the fundamental field, due toits higher frequency and, therefore, shorter wavelength.

Thus, the 3rd harmonic field will be strong, relative to the fundamentalfield, at the cathode surface where it is required to properly gate theFE cathode emission. In the body of the gun, however, the fundamentalwill dominate, yielding dynamics similar to those of a conventional gun.

FIG. 8 illustrates FE cathode gun cell test geometry used forsimulations.

FIG. 9 illustrates the fundamental and 3rd harmonic fields, plotted asdistance along the axis of the gun.

Note that the fundamental field has twice the strength in the body ofthe cavity as it does at the tip of the cathode and that the 3rdharmonic field is twice as strong at the cathode tip as it is in themajority of the body of the cell. Therefore, for equal fields at thecathode tip, the 3rd harmonic is ¼ as strong in the body of the cell.With α=0.4, then, the fundamental is a factor of 10 stronger in the bodyof the cathode cell. This meets our requirement that the fundamentalfield dominate the beam dynamics in the main body of the cell.

This is not an ideal process; in particular, the beam energy spread ishigher than desired, and further manipulation advantageously isperformed to make the beam more generally useful. However, this is trueof both photocathode and thermionic-cathode electron guns. Thesignificant advantage here is the ability of the FE cathode gun toproduce a beam that can be so manipulated, potentially in a packagewhich is superconducting, and thus makes extremely efficient use of theavailable RF power. These manipulations are fairly routine.

The examples of FIGS. 8 and 9, and the following sample calculations,are based on the choice of a 1.3 GHz fundamental RF frequency, with acorresponding 3rd harmonic at 3.9 GHz.

This particular choice of fundamental frequency was driven by threeconsiderations. First, there are several commercial RF power sourcesavailable in the range needed for the e-microscope application asillustrated in FIG. 10. Second, L-band cavities are of a size that is agood compromise between machining tolerances, where lower frequenciesare better, with compactness. Finally, the TESLA superconductingaccelerator structures are designed to operate at L-band, so there isalready a large and growing community knowledgeable about makingsuperconducting cavities, and associated systems, in this frequencyrange. In brief, it should be understood that this particular choice offundamental frequency simply is a convenient first choice.

It should be understood that the present invention is not limited tothis selection of frequency. Considerations exist and arguments can bemade for going to either lower or higher frequencies. It should beemphasized and understood that the FE cathode gating method of thepresent invention will, in general, operate independently of the choicefor the fundamental frequency. This is the addition of harmonic fieldswith a defined relationship in phase; therefore, everything scales withthe fundamental frequency. This includes, for instance, the bunchlength, which with longer (shorter) frequency will become longer(shorter) in time, but which will have the same length when expressed interms of degrees of RF phase. This has important implications for beamdynamics also, as it means that the basic performance should bemaintainable across a broad range of frequency choices. The ability ofthe cavity to properly support and accelerate a given beam current doeschange somewhat with frequency, but in general is more limited by theavailable RF power than by the particular design of the cavity or choiceof resonant frequency.

These calculations also do not incorporate some of the advanced cathodedesigns, such as, in particular a planar focusing cathode of theinvention as illustrated and described with respect to FIGS. 11, 12, and13. Such planar focusing cathodes are designed to help counter strongspace-charge forces acting on the beam as it leaves the cathode. Thesample applications below typically assume very low bunch charges; themodest average beam current comes from every bucket being filled (i.e.one bunch generated per RF period) and the high beam power from thecombination of moderate current and high beam energy.

For higher-current applications, such as for a free-electron laserdriver, it is anticipated that the planar focusing cathode of theinvention advantageously can be combined with the FE cathode gatingtechnique of the invention. It should be understood, however, that boththe planar focusing cathode of the invention and the FE cathode gatingtechnique of the invention represent different basic technologies andtechniques and should be considered independently on their own merits.

FIG. 10 illustrates an exemplary application of the gun of theinvention, for example, as illustrated in FIGS. 1 and 8 in accordancewith the present invention. The initial goals for this design were basedon the needs for electron microscopy. Thus, emphasis was placed onreducing the beam emittance (i.e. improving transverse quality) andenergy spread, while generating modest beam currents. For thesesimulations, the chosen bunch charge was 0.385 pC, or an average beamcurrent of 0.5 mA if an electron bunch is produced every RF period. Theelectron charge distribution was generated initially according to theprofile shown above, and later approximated by a Gaussian distribution.

In FIG. 10 there is shown an exemplary simulated beamline layoutgenerally designated by the reference character 1000 in accordance withthe present invention. An energy filter 1002 introduces a correlationbetween the beam energy and position, allowing a narrow slice to betransmitted from the core of the beam. This results in both a reducedenergy spread and an improved transverse quality, because the core ofthe beam generally is the portion where the transverse quality ishighest. PARMELA was used to simulate the entire beamline. A Gaussianlongitudinal distribution was used as a surrogate for the actual FEcathode emission profile, for ease of scaling to larger particle counts.The electron gun 100 coupled to the energy filter 1002 is amultifrequency FE cathode gun as described above. A cathode 0.1 mm indiameter was assumed, generating an initial beam current of 0.5 mA onaverage. The applied fields were as those above, with a peak field onthe cathode of about 25 MV/m. The energy filter was set to transmitabout 20% of the beam current, or 0.1 mA. Finally, a first-harmoniclinearizer 1004 reduces the beam energy spread by 2 orders of magnitudeand a third-harmonic linearizer 1006 reduced the beam energy spread byanother order of magnitude.

At the end of this simulated beamline, the beam current is about 90 μA.The average beam energy is 1.786 MV. The root mean square (RMS)fractional energy spread is 1.7·10⁻⁵, or about 30 volts in absoluteterms. The horizontal and vertical normalized emittances are 1.2·10⁻³and 1.0·10⁻³ μm, respectively. The difference arises because the energyfilter 1002 bends the beam in the horizontal plane. This should besufficient to generate a beam spot about 1 nm in radius, given goodelectron-beam optics. The total electron beam power is about 180 W. Thebeam power from the gun is closer to 900 W; the scrapers in the energyfilter absorb the difference. Therefore, the power density on at thespot could in principle be approximately 51 GW per square mm. Furtherreducing the transmission of the filter will result in additionalimprovements to beam quality, at the expense of current.

It should be understood that the present invention is not limited to theillustrated application of FIG. 10. For example, if the energy filter1002 is removed from the beamline, thereby passing all of the beamcurrent, the first-harmonic linearizer 1004 and the third-harmoniclinearizer 1006 can still be used to reduce the beam energy spread. Inthis case, the beam energy is around 1.4 MV and the final energy spreadis 1.7·10⁻⁴ rms (or about 300 volts). The beam energy is lower thanabove, and the energy spread is larger, because the energy filter 1002is not removing the “wings” of the incoming electron beam. Thus, adifferent minima for the energy spread is found. The transverse qualityis also worse, at about 4·10⁻³ μm. On the other hand, the entire beamcurrent of 0.5 mA is transmitted, for a final beam power of about 700 W.

As a comparison, a typical electron beam welder might have a beam powerof 15 kW, with a voltage of 60 kV. Thus, although the beam power ishigher, the e-beam welder's beam energy is lower by a factor of 20. Thebeam from the multifrequency gun 100 should therefore penetrate moredeeply into the material, and should almost certainly be able to providehigher-precision, smaller-area welds.

It should be understood that the beam power, 700 W, can easily beprovided for by relatively compact, CW RF power sources. This wouldresult in an e-beam welder that is smaller and more compact, due to theelimination of need for high-voltage DC power supplies.

Also if the cathode radius were to be doubled, to 0.2 mm, and the beamcurrent increased by an order of magnitude, to 5 mA, the final energyspread remains approximately the same at 1.8·10⁻⁴, and the emittanceincreases to 2.6·10⁻² μm, roughly in proportion to the electron beamcurrent. The beam power increases to 7 kW.

The penetration of an electron beam into matter scales (at low energies)approximately as:δz≈0.1·E ^(1.5)/ρ  (2)where δz is the penetration depth in μm, E is the beam energy in kV, andρ is the material density in g/cm³. This is an empirical formula, but isin reasonable agreement with theoretical calculations. For instance, a15 kV electron beam should penetrate about 2.3 μm into a silicatematerial with a density of 2.5 g/cm³.

Given a notional 100 kV beam energy for an electron microscope, the beamfrom the FE cathode gun, configured to run with the energy filter and afinal beam energy of 1.7 MeV, could be expected to penetrateapproximately 70 times as deeply into a sample, all other things beingequal.

For a typical electron beam welder operating at 60 kV, the expectedpenetration depth into iron or copper would be around 5.5 μm. (Actualwelds can go much deeper due to heat diffusion etc.) The beam from theFE cathode gun without the energy filter, with a final beam energy of1.4 MeV, should penetrate 0.6 mm, more than 100 times as deep, andtherefore depositing more of the electron beam energy into the volume ofthe metal as opposed to on the surface.

In brief, the disclosed method for gating the emission from afield-emission cathode makes the FE cathode a viable choice forhigh-brightness RF electron gun design. The beam quality is improved viastandard post-gun manipulations. Performance figures were calculated foran electron microscope; the results also indicate that a compact,precision electron-beam welder can be constructed using an almostidentical beamline.

Also when superconducting cavities are used for the gun and linearizercavities, there is effectively no power lost in the cavity walls and theRF power system can consist of relatively low-power, compact oscillatorsources. This would maintain a relatively compact footprint for anelectron microscope device, and should potentially reduce the footprintfor an electron-beam welder.

Other applications of interest include the use of the gun and linearizerto provide beam for a compact free-electron laser operating in the THzregion.

FIG. 11 provides a detail view of a novel cathode 102 that provides afocused electron beam in accordance with the present invention. Cathode102 is a planar focusing cathode. The planar focusing cathode includes aselected dielectric material 120, such as a ceramic material, to providean electron beam emission surface 130. A first metal surface 122 andconducting wall 110 respectively are provided both behind and in frontof the dielectric material 120 to shape the electric fields thataccelerate and guide the beam from the cathode surface 130. Thedielectric material 120 can be penetrated by electric fields, allowingthe planar focusing cathode 102 to provide focusing for the electronbeam starting at the substantially flat surface 130 of the cathodedielectric material 120. The first metal surface or shorting plunger 122behind the dielectric material 120 is slidingly positioned in a cavityor vacuum 124 relative to the dielectric material 120. The distancebetween the shorting plunger 122 and the dielectric material 120determines the effective focusing force applied to an electrode beam,for example, as illustrated in FIG. 13

FIG. 12 illustrates exemplary electric field contours of the planarfocusing cathode 1100 of FIG. 11 in accordance with the presentinvention.

FIG. 13 illustrates an exemplary normalized radial electric field at thecathode surface of the planar focusing cathode of FIG. 11 in accordancewith the present invention. In FIG. 13 a radial electric field at thecathode surface is shown that is normalized to the longitudinal field,as a function of radius, for three different positions of the plunger122.

Having reference now to FIG. 14, there is shown exemplary beamprocessing apparatus generally designated by the reference character1400 including the RF electron gun 100 for implementing beam processingmethods in accordance with the present invention. Beam processingmethods implemented with the novel high power, long focus electronsource 100 include e-beam welding and e-beam evaporation for thin filmdeposition.

In accordance with features of the invention, the RF electron gun 100with Field Emission Cathode Gating and Planar Focusing Cathodes of thepreferred embodiment is used as the electron source 100 for the beamprocessing apparatus 1400. The e-beam output of RF gun 100 indicated bya line 1401 is transported through a series of dipole magnets 1402 tosteer the e-beam and one or more quadrupole magnets 1404 to re-focus thee-beam. The focusing magnets 1404 position the e-beam centerline to anyselected position. A final steering magnet 1406 bends the e-beam tofocus on a target 1408, such as a weld joint, or deposition target.

In accordance with features of the invention, the electron source 100has a large focal length. The electron source 100 has a large range ofseveral meters in which the beam 1401 stays focused. The long focallength of the invented electron source 100 enables a single electronsource to be used for e-beam welding and e-beam evaporation for thinfilm deposition. In addition, with the e-beam advantageously steered viamagnets 1402, 1404, and 1406, and in particular the final magnet 1406,enables the e-beam to be transported inside a target part, allowingwelds to be made from the inside of the part, eliminating trapped voids.

In accordance with features of the invention, the invention is used tovapor deposit coatings on the interior of the target part that isimplemented with an identical electron gun 100 and using the samesteering and focusing magnets 1402, 1404, and 1406 as used for welding.The e-beam 1401 is focused on a target inside a component to evaporatematerial and coat the inside of the part. Thus, both the benefits ofe-beam evaporation and a single energy source 100 are realized. Thise-beam evaporation process is at least as effective and morecontrollable than known conventional processes.

In accordance with features of the invention, the series of steeringmagnets 1402 include a series of dipole magnets and the refocusingmagnets 1404 include quadrupole magnets. The final bending or steeringmagnet 1406 includes, for example, a dipole magnet.

While the present invention has been described with reference to thedetails of the embodiments of the invention shown in the drawing, thesedetails are not intended to limit the scope of the invention as claimedin the appended claims.

1. Apparatus for beam processing comprising: a high power, long focuselectron source for generating an e-beam; said high power, long focuselectron source for generating an e-beam including an RF electron gunhaving a planar focusing cathode for providing a focused electron beamhaving a selected dielectric material for providing an electron beamemission surface; said electron beam emission surface including asubstantially flat surface; a first metal surface behind said dielectricmaterial; a second metal surface radially surrounding said dielectricmaterial; said first metal surface and said second metal surface forshaping electric fields that accelerate and guide an electron beam fromthe electron beam emission surface; and said dielectric material beingpenetrated by electric fields and allowing the planar focusing cathodeto provide focusing of said electron beam starting at said substantiallyflat surface of the cathode dielectric material; a series of steeringmagnets transporting the e-beam for steering the e-beam and at least onerefocusing magnet for refocusing the e-beam; and a final steering magnetfor bending the e-beam to focus on a target.
 2. Apparatus for beamprocessing as recited in claim 1 wherein selected dielectric materialincludes a ceramic material; and said selected dielectric materialfunctions as a cathode of a high brightness RF electron gun and providesfocusing of the e-beam.
 3. Apparatus for beam processing as recited inclaim 1 wherein said series of steering magnets include a series ofdipole magnets arranged to steer the e-beam.
 4. Apparatus for beamprocessing as recited in claim 1 wherein said at least one refocusingmagnet to refocus the e-beam includes at least one quadrupole magnetarranged to refocus the e-beam.
 5. Apparatus for beam processing asrecited in claim 1 wherein said final steering magnet for bending thee-beam to focus on a target includes a dipole magnet arranged to bendthe e-beam to focus on a target.
 6. Apparatus for beam processing asrecited in claim 1 wherein said target includes an e-beam weldingtarget.
 7. Apparatus for beam processing as recited in claim 1 whereinsaid target includes an e-beam evaporation target for thin filmdeposition.